Electric induction motors are by far the most common, popularly used form of a.c. motor. They find ubiquitous application in refrigerators, air conditioners, major appliances, and a host of other machine applications. When fully loaded, induction motors may be designed to exhibit exceptionally good efficiency and quiet, long-term operation with negligible maintenance. The art of induction motor manufacture is so highly developed that a wide variety of motors are routinely made for all sorts of applications with such motors providing predictable performance characteristics and low unit cost.
While efficiency of induction motors may be readilly maintained at a high level when driving a full load, they also have notorious inefficiency problems when unloaded or lightly loaded. Ordinary induction motors literally waste a large percentage of their electrical power consumption as unecessary heat when they are delivering intermediate levels of output member (drive shaft) torque. It is this area of ENERGY waste which occurs under operating conditions that present less than a full load to the motor which has been previously addressed by several of my earlier inventions and remains the technical area which continues to be improved upon by my instant invention.
Refrigerations and air conditioners are two of the most prodigious producers of unecessary electrical ENERGY WASTE that, to a substantial extent, is caused by induction motor losses. As is well known, common hermetic compressors used in refrigerators utilize small induction motors ordinarily rated between about 1/6 and 1/2 horsepower for operation. Motor design is dictated to a large extent by engineering windings that develop sufficient magnetic field strength to produce adequate running torque in the motor under worst case conditions of high compressor loading, typical of extreme climatic conditions of heat and humidity. Obviously a motor carefully designed so as to be adequate for extreme climatic conditions will be considerably over-rated under milder conditions.
Domestic refrigerators, as a categorical induction motor application, are known to consume about 7% of the total amount of electric energy produced in the United States. More significantly, these same domestic refrigerators claim about 20% of the electrical consumption of the average household. As a result of this, a mere 14% improvement in refrigerator motor operating efficiency would unburden our nation's electric power grid by about one percentage point. Said another way, one out of every one hundred power plants could be "turned off" if this mere 14% efficiency improvement were in place in every domestic refrigerator. Air conditioners are even bigger energy hogs, particularly in summertime and in warmer regions of the nation. Energy consumption by air conditioners may dwarf all other uses, particularly when weather is severely hot and/or humid. It also behooves the layperson, if not the practicing artisan, to realize that the true mechanical load presented to the typical air conditioner motor varies widely, depending upon ambient air temperature, humidity, heat-load changes, and so forth. Again as with refrigerators and other appliance applications, induction motors intended for air conditioner use are intentionally over-rated (over designed) to accomodate "worst case" scenarios, while in fact they may ordinarily operate under conditions demanding much less motor running torque.
Operating an induction motor with magnetic field strengths nearing the stator core material saturation level is a common operating mode in modern motor design, where the central goal is to get the most torque for the least unit cost. High flux densities are often obtained from windings having a minimum of copper (e.g., reduced circular mil wire cross-section) in order to cut cost and weight, particularly with the advent of modern high temperature insulating materials. The net result is a motor field which runs hot and with low efficiency under all but full load running conditions. As is well known, when an induction motor is unloaded (e.g., the instant driven mechanical load ordinarily coupled with the output member is reduced or decoupled) it produces a corresponding decrease in power factor. While in theory this reduction in power factor with the current lagging the voltage phase by perhaps 30 to as much as 70 degrees or so can cut true power consumption (e.g., power actually drawn from the source), it must be realized that the apparent power (the product of voltage and current flowing through the motor winding) still remains high. An artisan familiar with the ramifications of power factor changes in an a.c. induction motor circuit knows that power draw from the a.c. line is of course reduced as the load coupled with the motor is reduced, but that the proportional reduction in a.c. line power wattage draw is not nearly so strong as what ought to be obtained in view of the extent of mechanical load reduction. What actually happens is that the "apparent" level of circulating current through the field windings continue to set up magnetic flux fields in the stator core which bring about almost as much eddy current and hystersis loss as what is produced when the motor is fully loaded. Additionally, this same current continues to produce resistive losses in the windings, a so called "copper loss" which results in a considerable level of heating sometimes approaching the heating that occurs when the motor is fully loaded.
An example quickly makes this problem apparent. A General Electric model 35JN23X motor draws about 6.6 amperes from an 117 volt source while providing 1/3 horsepower. Power factor is about 82%, and knowing that one horsepower equals 746 watts, the motor performance appears as: EQU ((117 v.times.6.6 a).times.82%)/100=633 watts EQU ((746 w/3)/633 w).times.100=39% efficiency EQU ((100-39%).times.633 w)/100=386 watts waste power
Under about 25% "partial load", this same motor continues to draw an apparent current of about 5.1 amperes albeit the power factor appears to drop to about 40%, resulting in about: EQU ((117 v.times.5.1 a).times.40%)/100=238 watts EQU ((746 w/3).times.25%)/238 w=26% efficiency EQU ((100-26%)/100).times.238 w=176 watts waste power
It would be better if the apparent current draw (said as 5.1 amperes) were considerably reduced when the load is light. This apparent level of current flows through the windings of the motor field, inducing magnetic fields in the structure. It is the energy returned by the inductance of the motor which lowers the power factor and keeps power draw down. However, this same level of apparent current flow produces copper losses in the winding in the form of "IR" heating losses. Additionally, the flux field induced in the field core produces considerable eddy current losses in the iron (silicon steel) making up core structure. Indeed, substantially reducing (such as "halving") the current draw under light or no load conditions is known to bring about considerable savings in both of these common areas of power loss. I have found that lower winding current under reduced motor load can be readily obtained by increasing the winding inductance. Such operation with increased winding inductance is correlational with having otherwise reduced the applied motor voltage coupled with an unmodified winding under conditions of reduced or no load.
As well known to artisans, the ampere-turn relationship of the winding is a principal determinant of magnetic flux levels in a motor's field structure. Therefore, a mere 10% increase in turns-count increases the effective inductance about 21%, and results in about a 10% decrease in overall ampere-turn excitation level. A BASIC computer routine may quickly show this relationship;
__________________________________________________________________________ 10 'COMPUTATION OF EFFECTIVE AMPERE/TURN EXCITATION LEVEL CHANGE 20 '(c) H. Weber 10/12/90 K1VTW MBASIC ATL-1.BAS 30 '- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 40 INPUT "Original Number of Turns ";NTA 50 INPUT "Additional Number of Turns ";NTB 60 NTX=NTB/NTA 70 LX=1/((1+NTX) 2) 80 AT=LX*(1+NTX) 90 PRINT:PRINT "AMPERE TURN level = ";AT;"% of ORIGINAL LEVEL" 100 'finis __________________________________________________________________________
In refrigerator and air conditioner applications, any unecessary power waste under reduced load compounds itself as an unecessary increase in overall system loss. This comes about due to the configuration of the motor, where it is hermetically sealed into a module integral with a compressor. What occurs is that the heat load of the motor inefficiency is contributed to the overall refrigerant system (e.g., the Freon gas loop) where it must be disposed of through increased condensor size and additional compressor effort. In practical effect, the overall size of any given refrigeration system is substantially upscaled to accomodate waste motor power.
In U.S. Pat. No. 4,266,177 for "Power Factor Control System for AC Induction Motors", Frank Nola described an early effort in obtaining a reduction of power input to a less than fully loaded motor. In this early work, a.c. power to the motor's main RUN winding was simply ON or OFF. Operation was akin to that obtained with a phase angle delayed electric lamp dimmer, aside from the control signal being derived from current lag changes (e.g., relative with power factor). In this NASA invention, the sudden inrush of triac turn-ON power introduced harmonic distortion of the a.c. power flow. This distortion was found to be objectionable by others attempting to practice the invention, and considerable parasitic loss (so-called third-harmonic distortion, in particular) was introduced into the utility power grid which was thought to offset any gains which might otherwise be produced by this invention. In any event, in a build-up of Nola's configuration as essentially taught in this earlier patent I found negligible power savings between having the controller ON or OFF (e.g., bypassed) when operating an ordinary 1/2 horsepower split-phase motor at partial load and with actual power consumption having been measured on a Westinghouse type CS watthour meter (as ordinarily used by utility company customers). I also found the motor operation erratic and noisy, producing buzz which was probably caused by triac turn-ON.
In yet another U.S. Pat. No. 4,533,857 for "Electrical Energy Savings Device for Motors", Ten-Ho Chang et al said that savings could be obtained by measuring motor current and providing phase-angle delay of the motor power turn-ON during each half-cycle. Unfortunately, common induction motors typically maintain relatively high levels of apparent current flow even when unloaded and certainly when partially loaded. Mainly, the phase lag of the current changes. In Chang's device, only apparent current is measured and thus the scheme is inapplicable to an awful lot of ordinary cheap induction motors. Additionally, Chang's device suffers the aforesaid shortfalls which afflict Nola: that being the losses introduced by partial cycle power flow caused by phase-angle delay of power turn-ON.
I have already taught advantages which reducing motor excitation levels under all but full load provide in terms of power savings and ENERGY CONSERVATION. In U.S. Pat. No. 4,806,838 "A.C. Induction Motor Energy Conserving Power Control Method and Apparatus" issued Feb. 21, 1989 I described a motor having two sets of parallel RUN windings. A main RUN winding set is fully energized by direct connection with the a.c. power source. Through engineered design, this main RUN winding set is ordinarily sized to produce just sufficient field flux to alone operate the motor under minimum load conditions. As motor load increases, additional power is introduced into a secondary RUN winding which is wound so as to contribute to the field flux produced by the main RUN winding and result in a stronger field. The power increase in the secondary RUN winding is related to motor loading, and full a.c. power is fed to the secondary RUN winding when the motor is fully loaded. I sampled current flow through the main RUN winding with effective motor loading being determined by instant phase lag of this current flow. In other words, increased loading produces a decrease in current phase lag. Although a special motor, having multiple RUN windings, is needed to implement my earlier invention, its benefits in power savings are substantial due to reduced eddy current losses and lessened winding losses under any running conditions less than that of full load. Unlike Nola and Chang, my invention maintains substantial power flow over the full 180 degrees of every a.c. power half-cycle even when less than fully loaded. The result is at least minimal, and usually nearly negligible levels of loss caused by a.c. power distortion.
In another U.S. Pat. No. 4,823,067 issued Apr. 18, 1989 for "Energy Conserving Electric Induction Motor Control Method and Apparatus" I have again taught the use of more than one parallel RUN winding acting in concert to modulate field flux in relation to instant motor loading. In its usual embodiment, my earlier invention employs two separate RUN windings wound effectively in parallel to produce additive flux contribution to the motor's magnetic field strength. In this arrangement of my earlier invention the first RUN winding is fully excited from the a.c. power source, with the ampere/turn design of the first RUN winding engineered to alone produce sufficient field flux to ordinarily operate the motor near full subsynchronous speed under minimum load. As motor load increases, subsynchrous speed decreases introducing an increase in motor speed slip. It is this increase in speed slip that is sensed and used to determine an increase in a.c. power which may flow to the secondary RUN winding. As before, when the motor is fully loaded immediate circuit operation is established to bring about full a.c. power coupling with both RUN windings thereby producing a maximal level of field excitation and a resulting full-torque operation of the motor's rotor coupled output member.
In yet another U.S. Pat. No. 5,013,990 issued May 7, 1991 for "Energy Conserving Electric Motor Control Method and Apparatus" I further teach a reactor coupled in series between an induction motor's usual RUN winding and a.c. power source. The reactor is sized to introduce some voltage drop, typically about 10-20%, and maintain sufficient magnetic flux level in the motor's main RUN winding to keep the motor running properly under reduced load. When loading increases, motor slip speed increases or conversely power factor increases, signalling for an increase in applied a.c. power. The increase is instantly produced by shunting the voltage drop developed across the reactor by turn-ON of a triac that is in parallel with the reactor. I do show that the reactor might have one or more taps, and as such the level of instant motor power might be tailored to suit the immediate motor loading conditions. Ordinary practice of this invention requires the use of an inductor (e.g., a reactor or choke) separate from the motor, and it is the reactive voltage drop which develops across the reactor due to current flow to the motor RUN winding which produces a reduction of motor terminal voltage. As a result, the instant level of motor terminal voltage may undesirably decrease in response to increases in motor loading.
In a co-pending application Ser. No. 07/237,045 filed Aug. 29, 1988 for "Energy Conserving Electric Motor Control Method and Apparatus" I continue to describe an induction motor having a main RUN winding and a supplementary RUN winding. The main winding is fully excited by a.c. power, providing just enough magnetic flux in the field to operate the motor while driving a minimal level of mechanical load. A program ordinarily is defined for the motor's operating cycle and proportionately more or less power is simultaneously fed to the supplementary load winding to provide additional field flux necessary to overcome anticipated changes in load. Ordinarily, a microprocessor or mechanical timer device may be used to operate the motor through any predetermined cyclic sequence, while some modulation of instant levels of the programmed power changes may further be obtained by sensing real-time fluctuations in motor load.
In each of these earlier patents as well as in one of the co-pending applications, my inventions entail novel utilization of an induction motor having two usually parallel-wound mutually coupled disimilar sets of RUN windings. Ordinarily, the first or main RUN winding set is wound with 10-20% more turns than the secondary or supplementary RUN winding set. This results in increased inductance in the first RUN winding set, and reduced current. The net result is substantially reduced ampere/turn excitation of the field by the first RUN winding set. Fabricating multiple windings in an induction motor is not unusual manufacturing practice in that multi-speed motors such as a General Electric model 7HR144S (1/2 hp 1725/1140 rpm 2-speed) are well known. However in these kinds of earlier designs, each RUN winding set is wound in a relatively different position (i.e., wound with angular displacement between the winding sets). For example, in this mentioned General Electric motor one winding set is positioned every 90 degrees as a 4-pole motor configuration, while the other winding set is positioned every 60 degrees as a 6-pole motor configuration. Clearly it is unusual to over-wind more than one disimilar turns count RUN winding in the same position as called for in my earlier inventions. I do strive to keep the accumulative amount of copper about the same, because my duplex main and secondary RUN windings are wound with wire having a guage substantially smaller than what a usual monowinding requires.
In smaller motors (fractional horsepower induction motors for example like those used in refrigerators, window size air conditioners, and major appliances) utilizing multiple RUN windings such as described by my earlier inventions it became apparent that it is sometimes difficult to stuff a sufficient number of multiple winding wire turns through stator corepieces of ordinary design. This condition is particularly aggravated by "insulation buildup" on the additional turns of wire, albeit the actual amount of copper involved remains about the same in either case. Recognizing this drawback, particularly in relation to mimimal redesign of pre-existing motor structure designs, I have found that utilizing a singular winding which is initially "over wound" with sufficient end-to-end turns to operate from about 10-15% higher than design center voltage results in a motor configuration which may be readily provided with at least one tap that matches the motor to the available a.c. power voltage level and manufacture may proceed without undue complication. Alternatively, of course a motor of standard design may merely have about 10% more turns added to one end of the original winding, with the juncture serving as the "tap", and preferably with the additional turns evenly distributed over each of the several field poles. Realized also is that the wire guage in either of these configurations, at least between the common end and the tap location, must be sized sufficient to carry the full operating current of the motor drawn with line voltage applied to the tap position while the remaining turns between the tap position and the end extreme from the common end of the winding may be of substantially smaller gauge. Common art practice teaches parallel connection of motor field windings, making tapped winding facture merely an extension of old practice. Take for example ordinary 2-pole induction motors rated for 117 or 234 volts: when connected for 117 volt operation, the two field windings are parallel connected. Primarily the advantage of this configuration over my earlier work using multiple RUN windings is that much less insulation build-up is encountered in the winding core windows and obviously less turns-count is required.
Induction motors having tapped field windings are well known in the art, but for purposes alien to the fundamental purpose of my invention. Such motors, like a McGraw-Edison model 203PEG, Emerson Electric model RAK-5107, Westinghouse model 322P490, and General Electric model 5KCP39DGA931T all have tapped field windings intended to obtain speed adjustment ordinarily with the motor directly coupled with a fan blade. Since torque demand of a fan changes in proportion to speed, reducing available motor torque causes increased slip in the motor that eventually reaches a point of equilibrium where fan speed matches available motor torque. Such motors are most common in 4 and 6-pole permanent split capacitor (PSC) arrangements, as in the case of an Emerson Electric model RAK-8558 used in Whirlpool air conditioners which runs about 1,075 r.p.m. at "full speed". This represents about 10% slip and is typical of these kind of known motors. Ordinarily, engineering goals have designed these kinds of motors to normally operate with "high slip", illustrated by trade motors such as a 4-pole General Electric model 5KCP39PGB810S 4-speed PSC motor having about 10% slip as rated for 1,625 r.p.m. full-speed or a 1/3 hp Emerson Electric model K-4340 PSC motor rated for 1,420 r.p.m. having about 21% slip as used in certain Frigidaire applications.
In contrast, good quality "constant speed" motors like a Westinghouse model 312P417, Emerson Electric model 3874, and General Electric model 37NN6X operate with merely about 4% slip. Aside from these general purpose motors, "low slip" motors are also widely used in "sealed" air conditioner and refrigerator hermetic compressor unit applications, where they commonly operate around 1,725 r.p.m. and 3,450 r.p.m. from 60 hertz a.c. power.
The astute artisan recognize that I have found a novel combination of the advantages afforded by several of my earlier efforts. I bring additional reactance into play in this invention which is much like having the external reactor of my earlier copending '079 application, but without the bulk and inconvenience together with expense of the separate inductor. By switching between the motor RUN winding taps as I now do, I alter the effective motor RUN winding circuit inductance (much like selectively shunting the reactor in the copending application) and I obtain truly efficient motor power flow under a wide range of external load conditions.